Chicago Transit Authority officials say the agency had to hand out more than 15,000 free rides after 165 fare readers malfunctioned at 60 rail stations.
CTA spokeswoman Tammy Chase says the problem, which occurred during the Wednesday evening rush hour, was linked to a "server failure" that resulted in some readers not working.
The failure occurred hours after CTA President Forrest Claypool said the performance of the Ventra fare collection system had improved from past glitches. The system was created by Cubic Transportation Systems, which received $454 million to create it.
Since its introduction in September, CTA riders have complained about double-billing, malfunctioning readers and terrible customer service.
Claypool has said Cubic won't be paid until billing and fare-reader problems are eliminated and customer service improves.